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Geological  Factors of Indoor Radon in Wisconsin

The indoor radon distribution map with amounts of radon represented as mountains

Left:  The indoor radon distribution, presented as “mountains”.  This map was produced from the same radon information by zip code mapped in blue, red and yellow at our website home page.  Altitude points, representing percentages of homes exceeding the EPA guideline for indoor radon, have been placed at the centers of zip code regions, and then processed into a relief map, shown with shading as if the sun were to the north-west.

Map of Bedrock Geology (Color) and Indoor Radon (Shaded Relief).  Click for larger image.

 

In the map of geology and radon, can you see one or two associations of indoor radon with certain bedrock types?

In the radon measurement summaries used for this map, one can see that elevated indoor radon is found in every region of Wisconsin, more in some regions than in others.

The radon mountains, based on zip-code data summarized as altitudes at zip centroids, are a crude approximation of the actual indoor radon distribution.  But they have the advantage that they can be shown in one map along with the bedrock type, allowing for direct visual perception of some associations.  One can see zip code boundaries in the color map of indoor radon by zip-code.  Zip codes are better for this purpose than one would guess at first, because regions of high population and high data density tend to have small zip-code areas, and regions of low population and low data density tend to have larger zip-code areas, causing the statistics for each altitude point to be better than they would be using a grid of uniform areas.  

Underlying bedrock is an indirect factor, and just one of many factors that can affect indoor radon concentrations.  Radon, a gas, can diffuse only a meter or two through soil from where it is created by radium decay, before it is transformed into a chemically reactive atom by its own radioactive decay. The soil under and around a house is usually the source of the indoor radon. 

  • Soils are derived by weathering of bedrock through eons of time.

  • Some soils have been transported by glacial action from the bedrock where they initially developed.

  • Radon comes from uranium through a long series of radioactive transformations. Uranium itself (half-life 4.5 billion years) is primordial, created in stellar supernova explosions along with many other elements (iron, nickel, silicon etc.) in interstellar dust from which the earth coalesced gravitationally.

  • Although rock types tend to have characteristic uranium concentrations, the uranium content can vary from place to place within a rock type.

  • Uranium, radium and other elements in the decay chain can be moved geochemically, for example by dissolving in water and concentrating where they come out of solution.

  • House construction (openings to soil) is another variable factor of indoor radon.


Map of Waukesha County Bedrock: Carbonates and Shale (0m)

map of Waukesha County Pleistocene (Subsurface Materials)

Credit for the maps:  Prof. Michael G. Mudrey, Wisconsin Geological and Natural History Survey, retired

Last Revised:  August 11, 2014